I have noticed that when i try to scroll up on the chat i can sometimes miss messages as it scrolls up too far, and when i hav expanded the chat box i have made it expand too much so i cant see the top of it to be able to lower it down again.
I suggest using the arrow keys on the keyboard to look up and down the chat box, and left and right to switch between the chats as it will be a quicker and simple way :)